A nuclear-reactor vessel is typically set inside a cavity formed of structural and shielding material. During refueling, the reactor-vessel head is removed, and that part of the cavity that extends above the remaining section of the vessel is filled with borated water, which has the twin purposes of shielding personnel working on the refueling operation and essentially eliminating further reaction in the fuel assemblies being handled. For various reasons, including the desirability of ventilation of the cavity, an opening normally exists between the reactor-vessel flange and the cavity wall. Since the reactor-vessel flange and a shelf on the cavity wall both form part of the floor of a pool that is to contain the borated water, it is necessary to cover the opening between the flange and the cavity wall. The pool seal is the cover for this opening, and when the pool seal is in place, the borated water cannot flow through the opening.
The pool seal, which is typically a large integral ring made of stainless steel in order to avoid corrosion, is difficult to build because of its size and the fact that its entire sealing surface must be machined to make it smooth enough to provide effective sealing. The resulting structure is large and rigid and is consequently quite unforgiving of irregularities in the height of the surfaces upon which it is to rest. While an inflatable seal is normally positioned between those surfaces and the ring, machined surfaces must nonetheless be provided, because the inflatable seal does not easily conform to small irregularities in the sealing surfaces. Though the inflatable seal can conform to larger irregularities, the larger irregularities still cause problems. The ring itself is too rigid to conform to these irregularities, and this causes insufficient sealing pressure at the "valleys" of the sealing surfaces.
Perhaps the chief problem with the integral sealing ring is the fact that, due to its large size and weight, rather elaborate transportation means must be provided in order to move it from the factory to the site. This requirement usually necessitates that the sealing ring be transported at the same time as the reactor vessel so that the sealing ring can travel on the same transportation means as the reactor vessel. In addition to the added difficulty of transporting both the seal and the reactor vessel at the same time, scheduling problems result because the sealing ring must be finished before the reactor vessel can be sent. This, of course, reduces the ability of the factory to schedule its resources efficiently.